When a power and a communication circuit are operated in proximity, the power circuit may produce certain conductive or inductive effects, which may interfere with the normal operation of the communication circuit. These electrical interference effects, which appear as a result of extraneous voltages and currents in the communication circuit, may be minimized by measures that are applicable to either circuit alone, or to both.

Such measures provide the basis for the coordination of power and communication circuits to avoid interference. Interference and Coordination Definitions of interference and coordination as adopted by the National Electric Light Association and Bell Telephone System lcs), with slight rephrasing, are:

Interference is an effect arising from the characteristics and interrelation of power and communication systems of such character and magnitude as would prevent the communication system from rendering service satisfactorily and economically if methods of coordination were not applied.

Coordination is the location, design, construction, operation, and maintenance of power and communication systems in conformity with harmoniously adjusted methods which will prevent interference.

The electrical-coordination problem arises principally because two distinct types of circuits or systems are employed, namely, (1) power systems for generation, transmission, and distribution of electrical energy, and (2) communication systems in which electrical energy is used incidentally for the transmission of signals.

Another important consideration arises from the fact that the user of electrical energy is generally also a user of electrical communication. For example, power lines for delivering electricity to homes and factories are roughly paralleled by telephone circuits required to give electrical communication for the same places.

The coordination problem becomes cumulatively more severe as the power systems supply increasing amounts of load and the communication systems become increasingly sensitive. There is also the complication caused by the introduction of neer uses for electrical energy and for electrical communication.

The effects of extraneous voltages and currents on communication systems are varied in character, and include hazard to persons, damage to apparatus, and interference with service. The damage to the physical plant includes the effects resulting from overheating, from brcakdown of insulation in lines and apparatus, and from electrolysis.

The interference with service includes such effects as noise and acoustic shock in the telephone circuits, false signalling in telephone, telegraph, and supervisory control circuits, as well as disruption of service.

Communication circuits are usually equipped with devices that, when subjected to excessive voltages, provide protection, but in so doing may render the circuit inoperative for communication purposes not only for the duration of the abnormal voltage condition but also until maintenance work can be done.

The coordination problem is extremely widespread; practically every type of electrical circuit has interfered
with some other type of electrical circuit. For example,power-supply circuits have interfered with audio- and carrier-frequency telephone and telegraph circuits, machine- switching and supervisory-control circuits.

Similarly, d-c and a-c railway circuits have interfered with practically every type of communication circuit,. It is an interesting and significant fact that communication circuits interfere with one another, not only in the form of “cross fire” between telegraph circuits but also in the form of “crosstalk” between telephone circuits on the same pole line.

Power circuits can interfere with each other. For example, a ground fault on a transmission circuit can impress high induced voltages on a neighboring low-voltage distribution circuit and produce apparatus failure or circuit outage.

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